Some chemical reactions can be challenging to conduct to high conversion of reactant(s) or high selectivity to a desired product when one of the reactants, or a desired product, or both a reactant and a desired product exhibit a high reactivity to form one or more undesirable byproducts. To overcome this challenge, chemical reactions are often conducted via alternate pathways in order to avoid the overly reactive reactant(s) and/or to avoid a high concentration of desired but reactive product(s). Yet other alternative pathways include using a multiple reaction sequence, as opposed to a one-step or one pot reaction. Often, alternative pathways require complex reactor designs. Yet other alternative pathways operate at low single-pass conversion to less than optimal yield, which then requires recycling and multiple passes to achieve an acceptable yield of desired product(s). All of these alternatives result in large, complex operations that add significantly to the cost of producing the desired product(s).
As an example, in the conversion of methane to methanol, direct selective oxidation is challenging, because one of the reactants, namely oxygen, is overly reactive with methanol. The desired reaction shown in Equation 1,CH4+½O2→CH3OH  (1)is not only exothermic, which has its own difficulties, but is made more difficult, because oxygen reacts with the methane or methanol in secondary reactions to yield undesirable byproducts. Specifically, the reaction of oxygen with methanol can yield complete oxidation byproducts, namely carbon dioxide and water, as shown in Equation (2):CH3OH+3/2O2→CO2+2H2O  (2)Likewise, over-reaction of oxygen with methane can also lead to complete oxidation byproducts, as shown in Equation (3):CH4+2O2→CO2+2H2O  (3)Either way, the outcome includes excessive production of undesired products CO2 and H2O and generation of heat, which accelerates the rate of the undesired reactions resulting in large amounts of methane not being converted to methanol. Conventional fixed-bed reactors avoid some of these undesired reactions by operating at very low conversions, therefore requiring excessively large reactor volumes and recycle volumes in order to achieve a high yield of methanol. Generally, such processes are not commercially desirable.
Alternatively, industrial practice involves a first reaction step of converting methane to synthesis gas (hereinafter “syn-gas”), that is, a mixture of carbon monoxide (CO) and hydrogen (H2) by way of methane-steam reforming; followed by a second reaction step in which syngas is converted to methanol. This process, due to its highly endothermic first step, requires large reactors in order to reach economy of scales and also requires complex engineering and operations due to a requirement for large energy input. In view of the above, it would be desirable to discover a one-step methane to methanol process that operates at low temperatures and high yields of methanol while avoiding undesirable secondary reactions. More generally, other reactions of interest include direct one-step selective or partial oxidation of alkanes, alkenes, or aromatic hydrocarbons, whether linear, branched, or cyclic, to the corresponding alcohols. For example, butane-to-butanol and benzene-to-phenol are also of interest.
Another example involves the oxidative dehydrogenation of alkanes to alkenes, for example, the oxidative dehydrogenation of methane to form ethylene, according to Equation (4):2CH4+O2→C2H4+2H2O  (4)In this example oxygen is the overly reactive component. Ethylene is more valued as compared to the starting methane. Yield of ethylene is negatively impacted by undesired reaction of product ethylene with reactant oxygen, Equation (5), and overreaction of reactants methane and oxygen, Equation (6), resulting in this oxidative process being of limited commercial application for the reason that methane and ethylene react disadvantageously with oxygen to form complete oxidation byproducts.C2H4+3O2→2CO2+2H2O  (5)CH4+2O2→CO2+2H2O  (6)
Yet another example involves alkylation of isobutane with butylene (an olefin) to form isooctane, a high octane low emissions component of gasoline. Alkylation proceeds according to Equation (7):i-C4H10+C4H8→i-C8H18  (7)wherein the isooctane product comprises a mixture of any number of 5, 6, or 7 carbon-chain isomers, with 3, 2, or 1 attached methyl (—CH3) groups, respectively. The butylene olefin is the overly reactive component. Undesired reaction can occur between the butylene olefin and isooctane product according to Equation (8):i-C8H18+C4H8→i-C12H26  (8)with higher polymerization another possibility as shown, for example, in Equation (9):i-C12H26+C4H8→i-C16H34  (9)Moreover, undesired side reactions can occur in the polymerization of butylene with itself; for example, the dimerization of butylene is shown in Equation (10):2C4H8→C8H16  (10)
Given the aforementioned difficulties, the alkylation of isobutane with butylene is conducted commercially using a homogeneous liquid acid catalyst, either liquid sulfuric acid or hydrofluoric acid. In either case, the acid needs to be blended with the reactants, separated from the products, and cleaned before reuse. Moreover, make-up acid must be added to replace acid that was consumed. While this liquid acid process is practiced on a large scale, it presents unique challenges due to a need to refrigerate the reactors in order to prevent thermal run-away reactions and a need to handle large volumes of corrosive and hazardous acids. Recently some solid-acid alternatives have been proposed to replace the homogeneous liquid acid catalyst; but here too, olefin polymerization side reactions can block catalytically active sites in the solid-acid catalyst. To overcome these issues, especially those related to catalyst deactivation, a combination of expensive and complex catalyst formulations and complex reactor designs is required in order to allow for frequent or continuous catalyst regeneration.
The difficulties and disadvantages of the aforementioned examples can be found in other specific chemical reactions also characterized by an overly reactive reactant, an overly reactive product, or both overly reactive reactant and overly reactive product, which participate in secondary reactions to form undesirable byproducts.
FIG. 1 illustrates a conventional prior art fixed-bed reactor wherein a single tubular reactor filled with solid catalyst particles has an inlet at one end for feeding a reactant mixture (A+B) and an outlet at an opposite end for exiting a product mixture (P). In circumstances wherein one reactant or one product is overly reactive towards secondary reactions, a concentration of undesirable byproducts accumulates along the length of the reactor.
FIG. 2 illustrates another prior art reactor, as described for example in U.S. Pat. No. 7,550,644, wherein an overly reactive reactant, A, flows through an inner tube that terminates in a catalytic zone of larger diameter; while reactant B enters the catalytic zone via a separate inlet. Reactants A and B mix in the catalytic zone and react on the surface of a catalyst coating the inner wall of the catalytic zone to form product P. This apparatus likewise leads to an accumulation of undesirable byproducts along the length of the catalytic zone, resulting from secondary reactions of overly reactive reactant A or product P.
U.S. Pat. No. 6,977,064, as illustrated in FIG. 3, describes a reactor comprising two concentric tubes such that the inner tube is constructed of a permeable material and the outer tube is constructed of a non-permeable material; and the annular space between the inner and outer tubes is filled with a particulate catalyst. A high reactivity reactant, A, is fed into the inner tube and passes at a controlled rate through the permeable material into the catalytic annular region, where it contacts a flow of reactant B to produce product P. The reactor provides for catalytic reaction through the entire annular region, which leads to an accumulation of undesirable byproducts along the length of the reactor resulting from secondary reactions of overly reactive reactant A or product P.
Finally, FIG. 4 illustrates another prior art reactor, as described in U.S. 2008/0234528A1, now U.S. Pat. No. 8,603,407, wherein two concentric tubes are provided such that the inner tube is constructed of a permeable material and the outer tube is constructed of a non-permeable material; and the annular space between the inner and outer tubes is hollow or empty space. Here, a catalyst is coated onto the inner surface of the outer tube, rather than being provided as particulates filling the annular space. The overly reactive reactant, A, is fed into the inner tube and passes at a controlled rate through the permeable material into the annular region, where it diffuses through a flow of reactant B towards the catalyst coating on the inner surface of the outer tube. Such an apparatus provides for reaction at the catalyst surface along the length of the outer tube, which can lead to an accumulation of undesirable byproducts resulting from secondary reactions of high reactivity reactant A or desired product P.
In view of the above, a need exists in the art for a reactor that reduces secondary reactions of one or more overly reactive reactants, or one or more overly reactive products, or an overly reactive reactant and an overly reactive product. Such a reactor would find use in providing a higher yield of desirable product(s) in circumstances wherein one or more reactants and/or one or more products are overly reactive via secondary reactions to form undesirable byproducts.